The present invention relates to a Fresnel step machining method; that is, a machining technique for forming Fresnel steps, each constituted of a refractive system prism and a reflective system prism, on a non-flat plate, and capable of performing this function at low cost. The technique is particularly adapted to the machining of a metal workpiece which will be used as a mold in the subsequent manufacturing of actual lenses
Conventionally, in a Fresnel lens in which Fresnel steps each constituted of a refractive system prism and a reflective system prism are formed on a lens member, the lens member is formed as a flat or curved plate (a spherical plate being mostly used in the latter case), as shown in FIG. 1. As shown in the case where either a flat plate or a spherical plate is used, (FIGS. 1(a) and 1(b) respectively), the configuration of the Fresnel lens is limited to a surface of revolution having an axis of rotation CL perpendicular to the face center C and coincident with the optical axis L of the Fresnel lens. In such a configuration, the Fresnel steps are formed as concentric circles with the axis of rotation CL as their center axis. In producing a metal mold for such Fresnel lenses, therefore, machining has been generally performed by turning the mold element on a lathe, and engaging the mold/workpiece with a tool fixed at a designed cutting angle.
Recently, however, lenses having complicated shapes which follow the external shapes of car bodies in order to reduce air resistance have been used for the parking lights, tail lamps and other lighting of cars and other vehicles, and lenses formed so as to have a quadratic surface or the like, with different curvatures in two directions (having no axis of rotation) have been required In the case where a Fresnel lens is to be formed with no axis of rotation, it is impossible to carry out the foregoing lathe machining technique in manufacturing the mold. Accordingly, a method has been used in which that portion having an axis of rotation and that portion having no axis of rotation are separately produced. The portion having no axis of rotation is produced by forming latticed Fresnel steps (angular direct vision steps) as shown in FIG. 2, this portion then being inserted as a core into the portion having an axis of rotation. In this three-dimensional Fresnel step machining method, however, there have been problems in that not only the number of machining steps becomes large, directly increasing machining costs, but the pitch of the Fresnel steps becomes large because of the use of a milling cutter. When the resulting Fresnel lens metal mold is used, the mold cooling properties are poor because of the core structure, so that defective moldings are apt to be generated.